In modern aircraft, the surface skins commonly take the form of composite laminates. Such composite materials comprise fibers embedded or impregnated in a resin matrix. The fibers are typically formed from carbon or glass. Often, it is necessary to attach to aircraft skins various components such as internal structural supports (i.e., stiffeners). In the case of a damaged aircraft skin section, repairs can take the form of attachment of a repair patch to the skin. In these and other instances adhesive bonding is usually desirable. The strength of the adhesive bond to composite materials is a function of the surface characteristics.
Commonly, the surface of composite materials has a glossy surface resin layer. This relatively smooth, mirror-like layer of resin is formed from excess resin which tends to flow and pool at the surface of the resin matrix once the embedded fibers are saturated during the curing process. Because the adhesive or bonding strength is a function of surface area, such a glossy surface exhibits a relatively low bonding strength.
As a result, there has been industry drive to develop low cost methods to increase the surface area of composite materials. One such development in the art is the use of peel-ply sheets during the manufacture of resin composite structures. A conventional peel-ply sheet consists of a sheet of woven material. The woven material is placed on the uncured resin surface of composite structures. The resin is allowed to saturate the peel-ply material. Subsequently, the resin composite with the applied peel-ply material is allowed to cure. Once the resin hardens, the peel-ply material is removed or peeled away from the resin composite. As a result, the surface of the resin composite is imprinted with texture characteristics similar to the peel-ply material. Accordingly, the surface area of the resin composite is increased. The textured surface of the resin, though being irregular, however, remains relatively smooth on a micro-level.
As such, whether or not a peel-ply procedure is utilized, further surface preparation techniques have been developed to increase the surface area of cured resin composites. A common technique is to perform manual sanding of the surface of the resin composite. As a result of sanding, the surface becomes roughened and the surface area is accordingly increased. Such an additional process, however, is labor intensive and therefore is relatively expensive.
Sand blasting or similar pressurized surface abrasion techniques are a more drastic approach to increasing resin composite surface area. While such procedures are effective in rapidly roughening the resin composite surface, controlling the degree of abrasion is difficult. Over processing the surface can result in damage to the resin composite structure. This occurs at areas where the surface resin is entirely removed and underlying composite fibers become exposed. Exposure of composite fibers results in the weakening of the structural integrity of the composite and is therefore highly undesirable. In addition, over processing can result in a polishing of the subject surface with a corresponding decrease of the surface area.
Moreover, with regard to surfacing processes, such as sanding, such processes only affect localized portions of the resin surface where a peel-ply application is utilized. Equating the textured surface to a series of mountains and valleys, sanding only effects the uppermost portions of the mountains. Thus, the surface at the lower portions of the mountains and the valleys would be unaffected by the sanding procedure. Thus, these unaffected regions would retain their glossy nature and would continue to exhibit relatively poor bonding characteristics.
It is therefore evident that there exists a need in the art for a low cost method of increasing the surface area of resin composites to improve surface bonding characteristics.